Electric discharge machining apparatuses perform machining by using the arc discharge generated in the electrode gap formed by an electrode and a workpiece to melt and remove material from the workpiece. With an electric discharge machining apparatus, surface roughness is refined by using finish machining conditions in which the machining current is sequentially decreased from a relatively large machining current that lasts for tens of microseconds and is used as rough machining condition to a pulse current that lasts for tens of nanoseconds and is used as fine machining condition.
Furthermore, for finish machining usage, consideration is given to use an RC circuit because the pulse width can be easily reduced. This circuit configuration includes an electrode gap formed by an electrode and a workpiece, a power supply that applies a voltage to the electrode gap, a current-limiting resistor that limits the electric current from the power supply, and an interelectrode parallel capacitor that accumulates discharge energy. Further, although it is not inserted into a circuit as an element, there is the floating capacitance of the capacitance components included in the mechanical structure and the cable. In the RC circuit configuration, when a power supply voltage is applied, an electric charge is accumulated in the interelectrode parallel capacitor and the floating capacitance in order to increase the interelectrode voltage. When the interelectrode voltage increases and an interelectrode distance decreases, interelectrode insulation is broken down, and the electric charge accumulated in the interelectrode parallel capacitor and the floating capacitance flows across the electrode gap. In the circuit configuration, the workpiece is melted and material is removed by the heat generated by the current.
Patent Literature 1 describes a technique whereby a magnetic circuit is connected in series with a tool electrode in an electric discharge machining apparatus. Consequently, according to Patent Literature 1, any discharge current due to the parasitic capacitance in a loop that includes a frame structure, a table, a machining piece, a machining zone, the tool electrode, an electrode holder, and a platform can be prevented from abruptly increasing, and thus the initial discharge current can be effectively attenuated.
Patent Literature 2 describes a wire-cut electric discharge machining apparatus in which a variable resistor is provided in wiring before a magnetic-switch contact, and an inductance element is provided in the wiring between the magnetic-switch contact and the workpiece. Consequently, according to Patent Literature 2, a circuit that supplies minute discharge energy to a machining gap is formed by the variable resistor and the inductance element, and the inductance element also has a role of negating the floating capacitance in the wiring.
Patent Literature 3 describes a high-frequency electric discharge machining apparatus in which an impedance matching circuit is provided between a high-frequency power generator and the start point of a coaxial cable and an impedance matching circuit is provided between the terminal of the coaxial cable and a discharge electrode. Consequently, according to Patent Literature 3, the output impedance of the high-frequency power generator is converted to the characteristic impedance of the coaxial cable so as to introduce the output power of the high-frequency power generator into the coaxial cable efficiently, and the output impedance of the impedance matching circuit can be regarded on the discharge load side as being a pure resistance, thereby enabling the electrostatic capacitance of the discharge portion to be reduced.